1. Field of the Invention
The present invention relates to a noncontact IC card reader/writer that supplies electric power and send data to a noncontact IC card and acquires receive data from the noncontact IC card by load fluctuation, particularly relates to a noncontact IC card reader/writer integrated with an antenna that reduces the effect of metal and others around an installation location and meets a stable reading characteristic and a stable writing characteristic.
2. Description of the Related Art
Heretofore, a reader/writer system using an IC card has been generally called a noncontact IC card system and is being practically utilized for a physical distribution system, a transportation system, an air cargo management system and others respectively utilizing a frequency band of 13.56 MHz for example. This system is provided with an IC card having an IC chip and an antenna coil on one card made of resin and a reader/writer that communicates with the IC card, and the reader/writer is provided with a loop antenna. Electric power and send data are constantly or intermittently transmitted via the loop antenna and receive data from the IC card located in a range in which the electric power and the send data can be received is acquired.
In case the antenna of the conventional type reader/writer is used near to metal, problems that a generated magnetic field is weakened by the effect of the metal, sufficient power supply to the IC card is disabled, communication distance is reduced and communication is disabled are caused. For a countermeasure, devices that a magnetic body such as a ferrite core is attached under the antenna, a magnetic circuit in which a magnetic flux easily wraps the front of the antenna coil is provided and the effect of metal at the back of the antenna is reduced are required.
FIG. 6 is a conceptual drawing showing a conventional type noncontact IC card reader/writer disclosed in JP-A-2002-298095 for example. As shown in FIG. 6, the noncontact IC card reader/writer 100 uses a flexible magnetic sheet 104 under an antenna 101.
FIG. 7 are explanatory drawings showing relation between whether a metallic obstacle exists in the vicinity of the antenna of the conventional type noncontact IC card reader/writer or not and a flow of a magnetic flux. As shown in FIG. 7, a reference number 101 denotes the antenna, 102 and 105 denote a magnetic flux, 103 denotes a metallic plate, and 104 denotes the magnetic sheet.
FIG. 7A shows a flow of the magnetic flux 102 around the antenna 101 and shows a state that no obstacle such as metal exists in the vicinity of the antenna 101. In this case, it is known that the magnetic flux 102 concentrically spreads with the antenna 101 in the center and magnetic density is high in the vicinity of the antenna 101.
FIG. 7B shows a flow of the magnetic flux 102 in case the metallic plate 103 exists in the vicinity of the antenna 101, when the magnetic flux 102 reaches the metallic plate 103, its magnetic field goes through the metallic plate 103, at this time, eddy current (not shown) is caused on the metallic plate 103 according to Ampere's right-handed screw rule, a magnetic flux in a reverse direction is caused at the back of the metallic plate 103 by the eddy current, acts in a direction in which the original magnetic flux is negated, and it seems that the magnetic field were disconnected by the metallic plate 103. Further, heat loss because eddy current flows on the metallic plate 103 is caused. Besides, when viewed from the antenna 101, the closer distance between the antenna and the metallic plate 103 is, the lower impedance is, the state is close to a short-circuited state, and the impedance of the antenna is extremely low.
It is a noncontact IC card reader/writer shown in FIG. 6 and disclosed in the patent document 1 that applies a magnetic circuit to avoid this phenomenon.
However, the conventional type noncontact IC card reader/writer has the following problems.
That is, in the case of the noncontact IC card reader/writer disclosed in the patent document 1, as shown in FIG. 6, the magnetic sheet 104 is merely arranged under the antenna 101 and only the magnetic sheet 104 which is a magnetic circuit is arranged between the antenna 101 and the metallic plate 103 as shown in FIG. 7C.
According to this structure, the magnetic flux 102 advances in a direction of the length in the magnetic sheet 104 to form a closed circuit, however, a magnetic flux 105 of a part passes the magnetic sheet 104, and emerges on the back side of the metallic plate 103. Particularly, an amount of passage immediately under the antenna 101 where magnetic fluxes concentrate is much, and the farther distance from the antenna 101 is, the more non-dense the magnetic fluxes are.
Therefore, the magnetic flux 105 that passes the magnetic sheet 104 reaches the metallic plate 103, loss by eddy current is caused on the large metallic plate 103 as described above, and the impedance of the antenna 101 is deteriorated.
In case the conventional type noncontact IC card reader/writer is directly installed on the metallic plate 103, the impedance of the antenna 101 is greatly influenced by the metallic plate 103 and varies because the magnetic flux 105 that passes the magnetic sheet 104 leaks, a resonance frequency fluctuates, the input/output impedance of the reader/writer and the impedance are not matched, and sufficient power supply from the antenna 101 to a noncontact IC card is disabled. As a result, problems that communication distance is reduced, communication is disabled and in the worst case, a transmitting circuit of the reader/writer is broken may occur.
Further, there is a problem that in case communication distance is extended, the size of the antenna 101 is required to be increased, naturally the size of the magnetic sheet 104 is also required to be increased, and a rate for which the price of the expensive magnetic sheet 104 accounts in the price of the whole reader/writer also greatly increases.